The duality of humankind has largely been visualized, at least in cartoons and broad comedies, as a tiny angel sitting on one shoulder while a devil is perched on the opposite shoulder. The inherent drama, of course, is watching as the character decides which symbol to listen to: the heavenly or the demonic. In the case of “The Mortal Instruments: City of Bones” (the ungainly subtitle is there because it’s based on a series of popular young adult novels, and because the filmmakers and financiers are convinced this will be the beginning of a long and lucrative franchise), this dilemma is writ very, very, very large. But the inherent tension of this decision is diffused by a painful amount of clumsily introduced back-story, thin characterization, and a pace that makes the “Twilight” movies, which this movie is clearly inspired by, seem positively speedy in comparison.

The movie opens by introducing us, very briefly, to Clary Fray (Lily Collins), an average Brooklyn teenager who goes with her romantically ambiguous BFF Simon (Robert Sheehan) to a nightclub, where she witnesses a murder that apparently only she can see. Soon enough, she starts doodling a mysterious symbol and is being followed by one of the murderers from the club, who she must admit is sort of dreamy. While being confronted by this stranger, who says his name is Jace Wayland (Jamie Campbell Bower, who comes off like the working man’s Nicholas Hoult), Clary gets a frantic call from her mother (Westeros’ own Lena Headey). Clary returns home to find her apartment ransacked and her mother completely gone. The mystery! The intrigue! The insurance claims!

Soon, Clary is drawn into the world of “shadow hunters,” half-angelic figures who fight evil but who cannot be seen by the “mundane” humans that populate the city. They walk around in lots of black leather, which makes them seem like the wimpiest biker gang ever, and every time they cast a spell, they have to etch a symbol into their skin, which leaves their flesh tattooed with runic graffiti. All of these impossibly young and pretty “shadow hunters” (including Kevin Zegers and Jemima West) make a pact to find Clary’s mother and retrieve some kind of magical goblet, a “mortal instrument” (just like the title of the movie!) that holds some kind of incredible power that we still don’t quite understand.

With a running time of 130 minutes, there’s certainly a lot of plot in “The Mortal Instruments: City of Bones,” but the story is so obscure and cluttered that if you pinned us down and asked us what actually happened in the movie, we would still draw a blank. There are tons of demons and werewolves and vampires and critters with slippery tentacles, although what the connection these mystical monsters have with the bad guy’s evil scheme is still beyond us. All of this nonsense probably would have made more sense if we had read the book (by Cassandra Clare) but that shouldn’t be a prerequisite for watching the adaptation. The movie should stand on it’s own. ‘The Mortal Instruments’ does not.

Collins, her hair dyed an unconvincing shade of red, does her best to emotionally anchor the movie in some kind of relatable reality. Her eyebrows, dark lightning bolts on her snow white skin, always seem to be twisted into some expressive punctuation mark. It’s just that her transformation from Brooklyn every-girl to bad-ass demon hunter, never feels authentic. The idea of a shadowy supernatural underworld just beyond our own is a tantalizing concept, and having an audience surrogate as charming and beautiful as Collins is a terrific idea, but the filmmakers don’t handle that transition gracefully. Instead of absorbing all of the weirdness with wide-eyed wonder, the narrative is clunky and awkward. Expository dialogue is less delivered than downloaded, and there’s nothing even remotely graspable on an emotional level. It’s all just a bunch of darkly lit hallways and poorly conceptualized, computer-generated creatures (the best of which look like ashy columns of smoke). Like “Buffy the Vampire Slayer” before her, this should have been a story of female empowerment. Instead it’s a story of a girl who, once again, does a lot of stupid stuff because she falls in love with a pretty boy.

Half of the fun of these young adult novel adaptations is seeing what adult actors the filmmakers recruit to give heft to the Abercrombie & Fitch catalog cast. Sadly, this is one of many areas where “The Mortal Instruments: City of Bones” feels hopelessly limp and listless. Besides Headey, we get C.C.H. Pounder as a low rent mystic, a horribly wasted Jared Harris as the leader of the shadow hunters whose main character trait is his uncanny ability to lurk on the outskirts of scenes, and, most laughably, Jonathan Rhys Meyers as Valentine, an evil demonic something-or-other who looks like some kind of S&M swashbuckler, complete with phony-looking mini-braids sewn into his actual hair.

The one highlight of “The Mortal Instruments: City of Bones,” besides trying to figure out if the guy who plays the good werewolf is Michael Sheen (it isn’t), is a moment when Clary suggests that one of the other shadow hunters doesn’t like her because he is also in love with Jace. Just the implication of homosexuality in one of these young adult movies feels almost revelatory, since these movies often dramatize teenage feelings of otherness by having characters who are aliens, vampires, or angels, but rarely ever feature an actual gay character. Elsewhere, characters brandish clear plastic weapons that resemble oversized sex toys and almost everyone dresses like extras from “Cruising” (just wait until you get a load of the “high warlock of Brooklyn”), all of which contribute to a knowing gayness that at least adds a sense of effervescence to a movie that, despite having actual jokes, feels dire and grim.

When, in the movie’s last act, Clary has to choose between the incredibly literal angel and devil, the drama is drained because the movie becomes bogged down in “Empire Strikes Back“-esque plot reveals (or are they?) and unnecessarily chaotic action set pieces, including some truly awful, “Birdemic“-quality computer generated birds. Director Harald Zwart, who also helmed the above average “Karate Kid” remake, shoots everything in luscious anamorphic widescreen, with lens flares and deep, cushy blacks. But no matter how good “The Mortal Instruments: City of Bones” looks, it’s hard to really care about anything that’s going on, and not just because we could barely understand it. [D]